Scale down of memory cells is proceeding with the trend toward higher degrees of integration in semiconductor memory devices, and noble metals such as ruthenium, iridium, and platinum are being investigated as electrode materials for memory devices. For DRAM devices, among memory devices, ruthenium is regarded as a promising electrode material because oxides thereof also have electrical conductivity and because ruthenium has excellent suitability for processing for scale down. An optimal method for producing a ruthenium-containing thin film for use in highly integrated memory devices is a CVD (chemical vapor deposition) method from the standpoint of the excellent step coverage thereof.
Organometallic compounds are thought to be suitable as precursors for forming a thin film therefrom by a CVD method, because the compounds are easy to handle. At present, (2,4-dimethylpentadienyl)(ethylcyclopentadienyl)ruthenium (hereinafter referred to as DER) and the like are known as organometallic compounds for forming a thin ruthenium film or a thin ruthenium oxide film therefrom (see, for example, patent document 1 and non-patent documents 1 to 8).
It is generally known that, of a precursor fed to the step of thin-film formation in a CVD method, the amount of the precursor which is actually used for thin-film formation is 10% or less, and that most of the precursor which was fed is discarded without being utilized. It is desired to recover and reuse the precursor which is being discarded.
The present inventors have found that the precursor recovered after the step of thin-film formation using DER as a precursor contains, besides DER, DER related structure compounds which were generated from the DER by the heating conducted during the step of thin-film formation and that the precursor, as recovered, cannot be reused.
The DER related structure compounds differ from DER in property concerning thin-film formation and, hence, the same thin-film formation as from DER cannot be attained. Because of this, there are the cases where the DER which contains DER related structure compounds also cannot be used to conduct the same thin-film formation as from DER.